This invention relates to a method of producing polyethylene naphthalate (PEN) utilized for recycling recovered PEN resin by incorporating it into the synthesis of PEN, and a recovered polyester-charging apparatus suitable therefor.
Thermoplastic polymer films are widely utilized as packaging films, printing plate support, films for printing, laminated films, support for photograph (movie film, X-ray film, etc.), support of magnetic recording medium or optical disc, or the like.
For example, polyester resin films represented by polyethylene terephthalate (PET) are excellent in dimensional stability, mechanical strength, transparency, etc., and used for fibers, films, sheets, molding materials, such as bottles, supports for photographs, supports for magnetic recording media, etc. A production of PET is disclosed, for example, in Japanese Patent KOKAI 7-207009 wherein polymerization is carried out with controlling vacuum degree of a final polymerization tank according to a specific predict model.
From the past, PET has been widely utilizing as the support of sheet-shaped image forming photosensitive materials, such as roentgen films and lithfilms. Off-specification products and trimmings generated by slitting both longitudinal sides of sheet have been recycled by chopped or pelletized through melt extrusion, and then as recovered material mixed with virgin PET.
On the other hand, polyethylene naphthalate resin having a higher strength has been developing for electric parts, support of magnetic recording materials, bottles and various films, and particularly, photographic film supports, bottles and the like have already been commerciallized.
As the production of PEN, there are the ester exchange method using 2,6-naphthalene dicarboxylic acid dimethyl ester (2,6-dimethylnaphthalate) and ethylene glycol, and the direct polymerization method using 2,6-naphthalene dicarboxylic acid and ethylene glycol as raw materials. In the ester exchange method, 2,6-dimethylnaphthalate and ethylene glycol are charged in an ester exchange reaction tank, ester exchange is conducted at 150 to 260.degree. C. with removing produced methanol as by-product to produce oligomers. In the direct polymerization, 2,6-naphthalene dicarboxylic acid and ethylene glycol are charged in an esterification tank, and esterification is conducted at 150 to 260.degree. C. under ordinary pressure or pressurized conditions with removing produced water as by-product to produce oligomers. In either method, the oligomers are transferred to a polymerization tank, and polycondensation is carried out under reduced pressure with heating while removing produced ethylene glycol as by-product (Japanese Patent KOKOKU 40-3964, 53-24233, Japanese Patent KOKAI 8-73576, etc.).
The production of PEN may be carried out either by batchwise operation or by continuous operation. In the case of batchwise operation, two tanks type, i.e. a combination of an ester exchange reaction tank or esterification tank and a polycondensation tank, is common, and in the case of continuous operation, ususally 3 to 5 tanks are used.
The PEN synthesized by polycondensation is, in general, extruded from a die having tens to hundreds holes several millimeters or less in drameter or a die for molding sheet having a slit, cooled, and cut into flake-shaped or pellet-shaped granules. The granules are conveyed to the subsequent molding and processing processes. However, the PEN taken out of the polycondensation tank can be extruded in strand or sheet, and treated by stretching, orienting, heat setting, etc. to be made into fibers or films, directly.
Although commercial articles of PEN are produced through the above synthesis, molding and processing processes, not all synthesized PEN become commercial articles. For example, in the case of molded articles, such as bottles, defectives generate by various causes, such as short shot due to oil pressure troubles of an injection molding machine or inferior temperature control of molds, roughened surface, contamination with bubbles, abrasions after molding, separation of printing and the like.
In the case of support for photographs and medium for magnetic recording, defectives and unutilizable parts generate, such as ununiformly stretched films generated in the beginning of film production, torn films caused by abnormal temperature of a stretching machine, and even under normal conditions, film edges utilized for grasping upon stretching, conveying or applying a reagent composition are slitted to generate trimmings.
Particularly, in the case of films requiring high quality, such as APS (advanced photo system, e.g. Shashin Kogyo, March, pp27-30, 1996) which is recently commerciallized by utilizing excellent properties of PEN, it was found to be industrially difficult of recycling by the conventional method taken in PET. That is, APS is required a great enlarging magnification, and its commercial value is greatly impaired by very small defects of image. Accordingly, white spots caused by foreign matter in film which shield light cannot be allowed.
Cellulose triacetate (TAC) used for 35 mm negative films can be dissolved in a solvent, and minute foreign matters which causes defects of image can be removed by filtering its solution prepared with a low viscosity at ordinary temperature. However, PEN must be melted for filtration because of no solvent, and it is a problem to filter melted PET having a high viscosity of 5,000 to 7,000 poise at a melting temperature of about 280 to 300.degree. C. for a short period so as not to degrade by thermal deterioration.
In the case of all virgin materials, it is possible a high precision filtration through a relatively small filtration area of a polymer filter, i.e. less deterioration by staying because contamination with foreign matters can be controlled upon manufacturing film. On the other hand, the life of polymer filter is remarkably shortened by using recovered materials containing foreign matters abundantly to degrade productivity. For the purpose of lengthening the life of polymer filter, to increase filter area results in the increase of staying period to accelerate thermal deterioration of the polymer.
Under the circumstances, it is possible to recycle recovered materials in a high grade, such as recovered materials from the support of photographic films, to the other uses, such as packaging films or molded articles, but such a recycling to lower grades is disadvantageous because of the reduction of commercial values. Moreover, in view of meeting the worldwide current of environmental protection, it is preferable to form a closed cycle in the same product, and it is desired to make a complete recycling in its own company without discharging wastes and by-products to the outside of the company.